This application claims the priority of German patent document 100 22 812.7, filed May 10, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for evaluating a traffic situation for a traffic network with traffic-controlled network nodes and roadway sections connecting them, based on traffic data obtained by reporting vehicles moving in the traffic.
Many methods are known for determining the actual traffic situation and for predicting the traffic situation to be expected in the future, in particular for road traffic networks. Such methods are becoming increasingly important due to the continuous increase in the amount of traffic. Conventional traffic prediction methods can be subdivided roughly into two types, namely historical progress line predictions and dynamic traffic predictions. The former are based on previously obtained traffic situation data from which an archive of so-called progress lines is formed; based on the latter a so-called matching process (in which a best matching progress line is selected) is then used to deduce the future development of the traffic situation from current traffic situation data. Dynamic traffic prediction, on the other hand, is based on identification of objects in the traffic and traffic states (such as free-flowing traffic, synchronized traffic and jams) from current traffic measurements, and dynamic tracking of these individualized traffic states.
These two prediction methods may also be combined. Such historical and dynamic traffic predictions are described, for example, in German Patent Documents DE 195 26 148 C2, DE 196 47 127 A1 and DE 197 53 034 A1, and German Patent Application 198 35 979.9. A necessary precondition for any traffic prediction method is to determine the actual traffic situation at the time of the prediction and, possibly, at earlier times.
Most conventional methods for traffic situation determination are applied to traffic networks in which the dynamics of the traffic flow are themselves governed essentially by the traffic interactions on the various roadway sections (the route connections between each pair of network nodes); that is, such dynamics are governed by the dynamics of the various identifiable traffic objects and phased transitions between them. Such interactions are applicable, for example, to high-speed roads.
On the other hand, different interactions occur in traffic networks in highly populated areas. There, the traffic flow is generally governed by the traffic control measures at the network nodes (for example, traffic lights at crossings), and scarcely at all by the traffic dynamic effects on the frequently relatively short roadway sections between the nodes. It is known that queuing theory can be used in these cases, in which the length of the queue before a particular traffic-controlled network node, the durations of the free phases during which the traffic is released at the relevant network node and interruption phases during which the traffic is stationary at the network node, the speed of the vehicles outside the typical queues before the network nodes, the inlet flows to the queue and the length of the roadway sections are of importance for the traffic dynamics. See, for example, S. Miyata et al., xe2x80x9cSTREAMxe2x80x9d, Proc. of the 2nd World Congress on Intelligent Transport Systems, Yokohama, Volume 1, Page 289, 1995 and B. Ran and D. Boyce, xe2x80x9cModeling Dynamic Transportation Networksxe2x80x9d, Springer-Verlag, Berlin, 1996.
German Patent Application 199 40 957.9 (not prior art) discloses a traffic prediction method which is particularly suitable for traffic networks in highly populated areas. This traffic prediction method is based on detection of actual traffic state parameters, which are formed in discrete time intervals by the free phases and interruption phases at the traffic-controlled network nodes, such as the actual vehicle outlet flow from a queue, the actual vehicle inlet flow into the queue and the actual number of vehicles in the queue. The actual traffic state parameters at discrete time intervals are used to determine effective continuous traffic state parameters, including at least one effective continuous vehicle outlet flow from a queue and/or one effective continuous vehicle inlet flow into the queue. From the latter, one or more traffic parameters is or are predicted on the basis of dynamic macroscopic modeling of the traffic. These include, for example, expected travel time at a prediction time for a specific roadway section and/or the expected traffic situation to be expected, at least with regard to the number of vehicles waiting in queues or traveling outside queues, and/or the predicted length of the respective queue. The contents of this prior Application with regard to the explanatory notes and definitions that can be found there of terminology and physical variables are also relevant here.
A parallel German Patent Application from the applicant discloses a method for obtaining traffic data by means of reporting vehicles moving in the traffic. This system is used to obtain what is referred to as FCD (floating car data), which is likewise especially suitable for traffic networks in highly populated areas (that is, for traffic networks in which the traffic is dominated by traffic controls at the network nodes). This method specifically includes obtaining FCD from dynamic individual or reporting vehicles, with such data including time stamp information denoting a reporting time which is not earlier than the time of leaving the relevant roadway section and is not later than the time at which the reporting vehicle reaches a next traveled roadway section before a next network node to be considered. Such time stamp information allows the routes traveled by the reporting or FCD vehicles to be tracked, and the travel times to be expected for the respective roadway section to be determined, possibly individually for each of a number of direction lane sets in this section. The term xe2x80x9cdirection lane setxe2x80x9d in this case denotes the number of different direction lanes in a roadway section, which may each comprise one or more lanes and are defined in such a way that the one or more lanes in a respective direction lane set can be used equally well by the vehicles in order to pass the network node to continue in one or more associated destination directions. This FCD traffic data acquisition method can be to determine travel times for each respective roadway section for the present traffic situation determination method, as used above.
One object of the invention is to provide an improved method of the type mentioned above, for determining one or more traffic parameters indicative of the traffic situation, using FCD information, particularly for traffic networks in highly populated areas as well.
This and other objects and advantages are achieved by the method according to the invention, in which traffic data indicative of the travel times on the roadway sections (that is, FCD suitable for travel time determination), are obtained by means of reporting vehicles moving in the traffic, and the travel for the roadway sections are determined from such traffic data. The roadway-section-specific travel times which have been determined are then used to obtain one or more traffic situation parameters. More precisely, these include the mean number of vehicles in a queue at a particular roadway section before a traffic-controlled network node, the mean number of vehicles in total on the roadway section, the mean vehicle speed on the roadway section before any queue (between the start of the roadway section and the upstream end of the queue), the mean waiting time in the particular queue and/or the mean vehicle density on the roadway section before the queue.
This method makes it possible to obtain FCD suitable for determining the actual traffic situation with sufficient accuracy, especially for traffic networks in highly populated areas where traffic dynamics are dominated by the traffic control measures at the network nodes, using the FCD for reconstruction. Other recorded traffic data (for example, from fixed-position detectors) can also be taken into account, but this is not essential. The actual traffic situation determined or reconstructed in such a way can then in turn be used as the basis for constructing a progress line database and, as a progression from this, for progress-line-based and/or dynamic traffic predictions. For predicting the traffic situation in a traffic network in a highly populated area, it is important to know the time-dependent queue lengths at the traffic-controlled network nodes, and the time-dependent number of vehicles on the respective roadway section. Such information can be obtained by the method according to the invention.
In one embodiment of the invention, the travel times and traffic situation parameter or parameters are determined separately, specifically for each of, possibly, a number of direction lane sets for a respective roadway section. This allows the accuracy of the traffic situation determination process to be significantly improved, since it takes account of the fact that queues of different lengths are generally formed for different direction lane sets before a traffic-controlled network node on a roadway section. Also, the traffic control at the network node is generally likewise direction-lane-set specific; that is, it includes different stopping and through-flow times, also referred to as free phases and interruption phases, respectively, for the various direction lane sets.
In another embodiment of the invention, the determined actual traffic information in the form of the one or more traffic situation parameters, determined on a roadway-section specific basis, and preferably especially direction-lane-set-specific, is used continuously for producing historical progress lines relating to the mean number of vehicles in the respective queue, the queue length, the mean waiting time in the respective queue and/or the mean number of vehicles on the respective roadway section.
In still another embodiment of the invention, the direction-lane-set-specific vehicle turn-off rate at a particular network node is taken into account as a further determined traffic situation parameter. That is, the method determines, for a particular time, how many vehicles, on average, are driving from a respective direction lane set of a roadway section entering an associated network node, via the node, into a respective direction lane set of a roadway section continuing on from that network node. This can be determined by means of suitably emphasized FCD; for example, the recorded FCD may contain information about the direction of travel or a change in direction selected at the network node.
In a further embodiment of the method, distinguished identification of the state of subsaturation on the one hand and supersaturation on the other hand is provided from a suitable travel time criterion. In this method, the determined travel time is compared with a threshold value which depends, inter alia, on the roadway section length, a typical free vehicle speed on that roadway section and the stopping and through-flow duration of the traffic control at the network node.
In a further refinement of the invention, traffic parameters are taken into account according to the method to be determined on the basis of different equation systems for the two situations of subsaturation and supersaturation.
A further embodiment of the method according to the invention allows specific, advantageous determination of the number of vehicles on a roadway section and of the effective continuous vehicle inlet flow into the roadway section and into a queue on that roadway section. Traffic data suitable for this purpose are available from two or more appropriate FCD vehicles which are traveling over the relevant roadway section with a time interval between them.
Another embodiment of the method according to the invention allows identification of the state of total overfilling of a roadway section (that is, a state in which the queue extends over the entire roadway section and possibly even farther upstream, beyond the network node there into other roadway sections.)
Another feature of the invention takes account of the inlet flow and outlet flow sources of vehicles as are formed, for example, by car parks and multi-storey car parks in inner city areas.
Finally, in the method developed according to the invention, a xe2x80x9cthinned-outxe2x80x9d traffic network is considered with regard to traffic situation determination, with a traffic network containing only a portion of all the roadway sections in an overall traffic network on which vehicles can drive, for example, only roadway sections of specific roadway types, such as major traffic roads. The other roadway sections are dealt with as inlet flow and outlet flow sources of vehicles.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.